It can be a common problem for a horse to refuse and even be terrified of crossing water.
The way you would want to help your horse overcome this fear is to break it down into steps. By breaking the process down into steps, you are going to help your horse gain a better understanding of what you are asking him to do.
Part of this process also involves you giving your horse something else to think about instead of looking at what is making him bothered and scared.
You are taking the role of a leader and giving him the confidence that he needs to cross anything.